1. Field of the Invention
The invention concerns matrices of photosensitive elements.
2. Description of the Prior Art
A standard way to make matrices of photosensitive elements is to provide for a network of row conductors and a network of column conductors with a respective photosensitive element at each intersection of a row and a column. A row of photosensitive elements, for which it is desired to know the output electrical signals, is selected through the network of rows. A respective output signal for each of the elements of the selected row is read through the network of column conductors.
The French patent No. 86 00716 describes a photosensitive matrix comprising a network of photosensitive dots in which each photosensitive dot has charge-storage capacitor in series with a photosensitive element, the unit being connected between a row conductor and a column conductor. The photosensitive element may be a PIN photodiode with three layers (P type semiconductor layer, intrinsic layer I and N type layer).
Charges are generated in the photodiode by the illumination of the photosensitive dot. They collect or accumulate at the (floating) node between the photodiode and the capacitor. They are read through the application, to the row conductor, of a pulse voltage in a direction that forward biases the photodiode (whereas it was reverse biased during the stage of accumulation). Charges, in a quantity corresponding to the charges that have collected, are then transferred from the floating node to the column (or conversely from the column to the floating node). The reading operation consists in measuring this movement of charges.
After the end of the reading pulse, the photodiode goes into reverse bias for a new stage of illumination and integration of charges.
However, the potential of the floating node no longer has the value that it had at the start of the integration stage. It is therefore not possible to start a new integration stage without setting this potential of the floating node at a well-determined starting value.
The reading stage is therefore followed by a stage for resetting the potential of the floating node.
The resetting is done by the illumination of the photodiode. It is therefore necessary to provide for an illumination source and control means synchronized with the means for reading the photosensitive matrix, to do a resetting after each reading stage.
An aim of the present invention is to make a photosensitive matrix wherein the capacitance of a column of photosensitive dots is reduced as far as possible so as to reduce the reading noise during the transfer of charges from a photosensitive dot to the column.
Another aim of the invention is to propose a matrix which is technologically easy to make.
Another aim of the invention is to reduce the dark currents which are injected into the column by the photosensitive dots of this column, even when these dots are not illuminated.
Another aim of the invention is to improve the efficiency of collection of the photocharges by the photosensitive diodes of the matrix.